The term “GaN-based” as used herein encompasses in particular all ternary and quaternary GaN-based mixed crystals, such as AlN, InN, AlGaN, InGaN, InAlN and AlInGaN and gallium nitride itself.
A fundamental problem in the fabrication of GaN-based light-emitting diode (LED) chips is that the maximum attainable electrical conductivity of p-doped layers, especially p-doped GaN or AlGaN layers, is not sufficient to achieve current spread over the entire lateral cross section of the chip with conventional front contact metallization, as known from LED chips made of other material systems (to maximize radiation decoupling, this type of metallization covers only a fraction of the front face).
Growing the p-type layer on an electrically conductive substrate, which would make it possible to impress a current over the entire lateral cross section of the p-type layer, does not yield an economically viable result. The reasons for this are as follows. First, the fabrication of electrically conductive, lattice-matched substrates (e.g. GaN substrates) for growing GaN-based layers is technically onerous; second, the growth of p-doped GaN-based layers on non-lattice-matched substrates suitable for undoped and n-doped GaN compounds does not yield adequate crystal quality for an LED.
In a known approach designed to combat the above problem, to effect current spread, either a contact layer permeable to the radiation or an additional layer of good electrical conductivity is deposited with substantially full areal coverage on the side of the p-type layer facing away from the substrate, and is provided with a bonding contact.
However, the first-cited proposal has the disadvantage that a substantial portion of the radiation is absorbed in the contact layer. The second proposal requires an additional process step that greatly increases production expenditure.